The present invention relates to a distance measuring device, as well as to a method for determining a distance and to a suitable reflective member.
Conventional distance measuring devices are used among other things, for example, for detecting the piston position of fluidic linear drives and pneumatic and hydraulic cylinders. The detection of the piston position on cylinders can be implemented both discretely, i.e. at discrete points, and continuously, i.e. constantly during operation.
A discrete piston position determination is generally required in order to report back the implementation or end of a piston movement to a sequence control system (e.g. SPS) in order to thus be able to initiate the next sequence step, for example.
Predominantly used for this purpose are sensors or sensor devices sensitive to magnetic fields which detect the magnetic field of a permanent magnet which is located on the cylinder piston. The sensors used here are fitted externally to the cylinder tube of the piston cylinder. If the piston moves into the detection range of this type of sensor, the latter recognizes the presence of the cylinder piston through the cylinder tube. For this, the use of non-ferromagnetic materials is predominantly required and so restricts the structural properties and applications of the drive.
If, however, a different position of the piston is detected, the sensor must be correspondingly mechanically adjusted. For each position to be detected, in addition a further sensor must consequently be fitted, accompanied by the associated additional material, fitting, adjustment and installation costs. This generally takes place at the customer's premises. Here the cylinder is often already integrated into a machine which is difficult to access, and adjustment of the switching distances by mechanically moving the externally fitted magnetic switches is no longer possible.
Furthermore, for these externally fitted sensors additional installation space is required. So that the accessibility and robustness of the sensor can be guaranteed, additional structural complexity is often required.
These types of sensor are predominantly in the form of sensors sensitive to magnetic fields and are known as Reed switches, magnetoresistive (MR), giant magnetoresistive (GMR), Hall switches or magnet-inductive proximity switches.
Complex coordination of the magnet to the sensor device is required for detection of the magnetic field. Moreover, with this measuring principle, the possible applications are restricted by interfering static and dynamic magnetic fields (EMV, field of a nearby cylinder) and the temperature characteristics of the sensor.
For the continuous measurement of the piston, position measuring systems are generally used which function potentiometrically, magnetrostrictively according to the LVDT principle (Linear Variable Differential Transformer) or according to the ultrasound principle. With these systems the piston position is emitted continuously and predominantly as an analog voltage signal. Sensors according to the LVDT principle always require a reference path when switched on. Magnetostrictive sensors are fitted either externally onto the cylinder or into a hollow piston rod. Both fitting possibilities mean substantially increased complexity, are prone to interference or weaken the stability of the drive in the case of the hollow piston rod. Ultrasound sensors are only suitable to a limited degree for the path measurement in pneumatic and hydraulic cylinders because the measuring accuracy changes with the cylinder pressure. Incremental path measurements are also known as a supplement to these systems. These systems are implemented, for example by the coding of the piston rod, and so can only be used for the relative path measurement.
Neither the continuous nor the discrete piston position determination can be integrated into a cylinder or can only be so with substantial structural complexity and the associated high costs. The substantial structural complexity is due to the fact that all of the established sensor principles described must be adapted to the corresponding cylinder length because the principles have a detection range which is too short.
The ideal path measuring system for determining the piston position in pneumatic and hydraulic cylinders has the following properties:                continuous, absolute path measurement with an accuracy of 100 pm for positioning the piston        total integration of the sensor with analysis electronics into the cover of the cylinder        switching distances should be adjustable externally via an electronic interface (teach-in capability)        a universally applicable sensor, independently of the cylinder length        measurement results independent of pressure, oil and humidity in the cylinder        reliable measurement results, e.g. up to 10 bar pressure and 6 m/sec piston speed in the pneumatic cylinder.        
In practice, known measuring systems patent application have the following problems for cylinders with a large diameter (>50 mm):                The plastic ring for the piston stop and the antenna retainer is very large. These plastic parts are only available up to a diameter of max. 60 mm as ready-made items. For larger diameters expensive custom-built models are required.        
Moreover, plastic absorbs water over time or releases water according to the conditions of use, and in this way changes the measuring conditions. The measurement results then become inaccurate and no longer correspond to the specification.
Classic end position damping e.g. of the pneumatic piston can only be achieved by the moved brake ring made of plastic at the expense of measuring accuracy.
Further disadvantages of known measuring systems are:
that the conventional pistons in the pneumatic cylinder are relatively thin and generally have a magnetic ring in the center in order to enable operation with externally fitted Reed switches. These pistons do not form an ideal reflective member for an electromagnetic wave. Part of the electromagnetic wave passes over the piston into the functional space of the cylinder lying behind this, returns with a time delay and interferes with the useful signal. This substantially worsens the measuring accuracy. Furthermore, there are pistons which are made entirely of plastic. These pistons do not constitute a reflective member at all for the electronic wave. The method described, for example, in DE 102 05 904.7 then no longer works at all.
Furthermore, a disadvantage of a piston stop in the cover made of plastic is that the plastic is settled by frequent piston impacts and so the physical conditions in the functional space of the cover change for the high frequency sensor. Moreover, the measuring accuracy worsens.
In addition, with smaller cylinder diameters it is very difficult to integrate the discrete electronics into the cylinder cover. Part of the electronics must then complexly be accommodated externally, e.g. on the cylinder wall.
A device for determining the position of a piston in a pneumatic cylinder is known from U.S. Pat. No. 4,588,953. During the movement of the piston a microwave signal is delivered into the cavity which is bordered by the outer wall of the cylinder and the piston. The reflected microwave signal is received and processed in order to determine the position of the piston.